I have gas fired Takagi tankless units installed at my home and rental properties. They can work well, but must be correctly sized and installed. I put up a web page about what we have learned at:
http://paragoninspects.com/tankless-heater-home-inspection-evanston-chicago-il-faq.html
http://paragoninspects.com/tankless-heater-home-inspection-evanston-chicago-il-faq.html